Graphic Flatpak Install

UNDER DEVELOPMENT

Installing in Ubuntu Bionics (18.04 LTS), Debian Buster (10 Stable) or newer versions like Debian Bullseye or Sid.

First you install some prerequirements for flatpak:

 $ sudo apt update
 $ sudo apt install flatpak
 $ sudo apt install gnome-software-plugin-flatpak

Or use synaptic to install flatpak and gnome-software-plugin-install

 $ flatpak --user remote-add --if-not-exists flathub \
       https://flathub.org/repo/flathub.flatpakrepo
 $ flatpak --user install -y flathub org.freedesktop.Platform//18.08

Note: since the flatpak commands are still on the Command Line I used the Command Line installation as well. Feel free to use a graphical tool like Synaptic instead.

From now, here is also a GUI method using Gnome Software:

Point the browser to http://opencpn.duckdns.org/opencpn/
Download the flatpak repository link

Open it using Gnome Software. This adds a new software repository.
Check that is is enabled using Software Repositories in menu top-right
Search for opencpn and install it.

EDIT: Note that these instructions are for the beta/nightly builds. The official release is planned to land in flathub.org, but later.